The drawbacks of the prior art shall now be presented in the light of the particular case of a known technique for applying a decision policy on mobility within an architecture of packet mode transmission networks.
A mobility manager applying inter-cell handover control by the network is presented in the document “Hierarchical Mobility Controlled by the Network” by Y. Khouaja, K. Guillouard, P. Bertin and J M. Bonnin in “Multiaccess, Mobility and Teletraffic for Wireless Communications”, Kluwer Academic Publishers, 2002.
This document defines a mobility manager who has the capacity to initiate and guide the execution of inter-cell handover, using information transmitted by the operator, the network and the mobile nodes. This mobility manager is situated in a cell network and enables the choosing of the target cells according to radio data (measurements sent back by the mobile units), network data (load, type of traffic, quality of service) and operator data (subscriber profiles, network parameters, activation thresholds, state of operation of access points). Each mobility manager manages a set of radio cells (i.e. a set of access points). The working of the mobility manager is described here below. The mobility manager (GM) transmits a list of the neighboring access points (PA) to the mobile node (mobile terminal). The mobile node (NM) transmits the measurements of quality of the radio link to the mobility manager. The mobile node asks the mobility manager to change the access point without specifying a new target access point. The mobility manager selects the target access point by consulting its database (BD). This database contains various pieces of information liable to assist decision-making in inter-cell handover. The mobility manager manages the change of access point in transmitting the data packets intended for the mobile node simultaneously to the two access points involved in the inter-cell handover, as soon as the mobility manager has knowledge of the imminent execution of the inter-cell handover. This duplication of data limits data packet losses.
The mobility manager described here above is used to apply mobility management based on rules that the network operator defines. This prior-art technique however has several drawbacks.
First of all, the centralization of a mobility manager has the drawback of making information go back by one or more hierarchical levels to the central point, thus increasing the application time. Furthermore, the inter-cell handover decisions are made for all the terminals that have to move. This makes the application time proportional to the number of terminals to be moved. With a centralized manager, the number of terminals is very great and therefore the application time too. Finally, the time taken to go through a decision tree is proportional to the number of pieces of information contained in the tree. Now, in a centralized manager, the number of pieces of information to be taken into account is very great.
Again, for the distribution of the rules, the defining of a deduction system (expert system) at each hierarchical level may be envisaged. However, this approach can not be used to take account of the decision rules in which the different categories of parameters (local or global) are mixed. In this case, the rules can contain only the local parameters. This approach requires the user to write rules that work only at one level and to distribute them by hand thereafter. In other words, the hierarchical structure of the management of the policy influences the writing of the rules of policy (in particular, when there is a change in hierarchical structure without a change in policy, the rules have to be rewritten). Furthermore, no means of decision tree distribution is described.